The invention relates to the preliminary treatment of plastic surfaces in order to prepare them for the application of chemical metal platings thereon. As is known, plastics cannot be readily metallized, and various methods are necessary for preliminary preparatory treatment of the plastic surfaces. The most commonly used pretreatment is that of pickling and activation, precedent to subsequent chemical and galvanic metallization. The pickling and activation are customarily effected either separately or in a single reaction stage.
Thus, for example, German Pat. No. 1,264,921 describes a method for the preliminary treatment of acrylonitrile-butadiene-styrene(ABS) copolymers, wherein the surface is treated in a single reaction stage with a solution of concentrated sulfuric acid containing an oxidant, such as chromic acid, and a noble metal salt. In one example of the patent, a value of 6% by weight of chromic acid is disclosed as a practical figure.
French Pat. No. 1,493,783 describes a similar method wherein a 5.4 to 18.45 molar sulfuric acid solution containing gold, silver, or palladium ions is used, to which up to 0.45 mole/liter chromic acid is added.
After pickling and activation, the plastic surfaces are rinsed with water, and then directly chemical metal plated, as for example with nickel, and then galvanized.
In the performance of the pickling-activation operation, it is customary to secure the plastic parts to be treated on racks. These racks are usually metal, e.g. steel, frames covered with insulation to protect them from the corrosive action of the pickling-activation bath. The insulation is usually natural or synthetic rubber.
While the salt of the noble metal, for example palladium, platinum, gold or silver, employed in the pickling-activating solution, may be any suitable soluble salt, such as sulfates, nitrates, acetate, or chlorides, the chlorides are preferred. These are usually added in quantities of about 0.01 to 1 gram of metal salt per 1 liter of chromo-sulfuric acid.
Experience has shown that where pickling-activating baths containing the noble metal in the form of chloride are employed, chlorine is liberated under the strongly oxidizing conditions present. This chlorine strongly attacks the rack insulation, degrading and oxidizing the organic materials therein, causing contamination of the pickling bath, and exposing the metal rack portions to attack by both the bath and the chlorine.